Accurate measurement and analysis of transpiration in herbaceous plants and trees is inherent in the effective growth thereof particularly under adverse environmental conditions such as in the desert and in underdeveloped countries. Unfortunately, the amount of water that is used by individual plants, shrubs and trees growing in indigenous fields, orchards, forests, or even urban landscapes, is not well known or documented.
Sap flow has been studied for many years by injecting heat into the stem of plants and trees. For example, the heat pulse velocity approach for measuring sap flow, suggested by Huber in the early 1930's. Another approach, the heat balance method, was suggested in the 1940's by German researchers and first implemented in Japan in the 1980's. See T. Sakuratani papers, "A Heat Balance Method for Measuring Water Flux in the Stem of Intact Plants" which was published in 1981 in J. Agric. Meteor. (Japan), vol. 37, pp. 9-17, and "Improvements of the Probe for Measuring Water Flow Rate in Intact Plants with the Stem Heat Balance Method" which was published in 1984 in J. Agric. Meteor. (Japan), vol. 40, pp. 273-277.
In U.S. Pat. No. 4,745,805, Granier teaches a process and device for the measurement of the flow of raw sap in the stem of a plant. More particularly, there is provided a process for measurement of changes in the f low of raw sap in the stem of a plant, which comprises steps for insertion of two temperature-monitoring probes, supply of an electric current of constant intensity to the heating probe, and recording of the temperature difference between the two probes. The two probes are placed in the same stem separated by a distance such that the heat released by the heating probe can not appreciably affect the non-heating probe. The non-heating probe is preferably placed perceptively on the same vertical line as the heating probe, but underneath it. The Granier device also comprises a heating circuit stabilized by the heating probe, a thermal couple measuring means (the hot and cold junctions which are placed respectively in the heating and non-heating probes), and means of recording the voltage at the terminals of the temperature-monitoring device. The heating probe comprises a rigid tubular core on to which is wound a heating wire, which is in turn surrounded by a heat distributing tube made of a high thermal conductivity material such as aluminum, with the winding of the wire and the distributing tube being of a length approximately equal to the thickness of the sap-wood into which the probe must be inserted, and the tubular core containing one of the supply wires of the winding.
U.S. Pat. No. 4,555,940, issued to Renger, discloses a method and apparatus for measuring and monitoring liquid flow rates and volumes in medical devices that pump liquids carrying medication into a patient's blood stream, spinal fluids, brain lymph glands and other organisms in the human body. The Renger apparatus includes a pyroelectric member for detecting or measuring changes in temperature of the fluid flow path and of the fluid as it passes through the temperature-modified part thereof. It is an aspect of this invention that the pyroelectric detector may be joined to the outside of the fluid flow path without penetrating therein, and thereby not impeding fluid flow therethrough. Particularly effective for tubular flow paths having a diameter in the range of 0.1 millimeter to about 5 millimeters, this apparatus measures and monitors both continuous and intermittent pulse liquid flow. The apparatus can verify the performance of the flow paths in tubular devices, warn of impending or existing unwanted changes in the rate of volume of flow, serve as a feedback system for monitoring flow at a predetermined rate or volume, detect bubbles, or some combination of these purposes.
More particularly, the Renger apparatus includes means for detecting changes in temperature as a function of fluid flow rates and volumes through the temperature-modified portion of the fluid f low path. This includes pyroelectric detector means such as pyroelectrically-sensitive polyvinyliden fluoride film. Pyroelectrically-sensitive film of this kind can be wrapped around the fluid flow path. For this apparatus to function properly, the pyroelectric detector means must be in close proximity to the temperature-modifying means in order to detect temperature changes reliably as the fluid flows through the temperature-modified portion of the fluid flow path. Preferably, the pyroelectrically-sensitive detector is joined to, and wrapped around, the fluid flow path. The detector should preferably be disposed upstream of the temperaturemodifying means. Where pyroelectrically-sensitive polyvinyliden fluoride film constitutes the pyroelectric means, the metal film electrode on the pyroelectric film itself can also serve as temperature-modifying means.
In alternative embodiments, this apparatus can either include temperature-modifying means adapted to deliver a fixed quantity of heat, to change the temperature of the fluid flow path by a fixed amount for a fixed period of time, or simply to stop when a fixed temperature change has been attained. The temperature of the fluid flow passage before temperature modification begins is considered to be the reference or zero point. The temperature of the fluid flow passage is then modified to a predetermined level before flow begins, and the temperature change is detected relative to the zero point. After fluid flow takes place to the temperature-modified portion of the fluid passage, the resulting temperature is detected as it relates to the fluid flow.
The energy balance method for measuring sap flow, well known in the art as the "stem heat balance" method, enables an absolute measurement of sap flow rate to be ascertained in the stems of intact plants or the trunks of woody species. See papers by T. Sakuratari published in J. Agric. Met. (Japan), vol. 37, pp. 9-17 (1981) and by J.M. Baker and J.L. Nieber published in Agric. For Meterol, vol. 48, pp. 93-109 (1989). According to this method, a stem section is continuously heated and the components of the heat flow are accurately measured. As known to those skilled in the art, provided that steady state conditions are sustained, this method affords accurate measurement of sap flow rates and inherently requires no empirical calibration but only a zero set determination before dawn. In addition, the energy balance method provides a continuous record of the sap flow rate and its accumulation over time. While the efficacy of this method has been demonstrated under controlled conditions in a greenhouse and the like, its applicability to diverse environments has been limited by the difficulty associated with isolating the sap flow gauges from the adverse effects of water intrusion, radiation, noise, etc. Thus, the prior art suffers from the inability to routinely and economically apply this methodology for measuring sap flow in the field.
Accordingly, these limitations and disadvantages of the prior art are overcome with the present invention, and improved means and techniques are provided which are especially useful for ascertaining sap flow in herbaceous plants and trees.